1. Field of the Invention
The invention relates to a displacement element having a cylindrical housing filled with a fluid under pressure and exhibiting two sections of different internal diameter, to which sections a piston capable of axial displacement is allocated in each case. Each of the pistons is adapted to the respective internal diameter of its section and dividing the respective section of the cylindrical housing into two pressure chambers. Each of the pistons has a piston rod penetrating the housing at an open end, which first piston is arranged at one end of the piston rod present in the housing and each of the two pistons possessing means for damping the movement of the piston rod respectively in one direction. The displacement element finds an application in pivotally mounted components, and in particular for the hoods or rear hatches of motor vehicles.
2. Description of the Related Art
A displacement element of this kind is disclosed in U.S. Pat. No. 4,230,309 which permits damping of the movement of the piston rods before reaching the end position of the retraction and extension movement. For this purpose, the displacement element possesses a cylindrical housing that is filled with a fluid under pressure. The cylindrical housing exhibits a larger internal diameter at its closed end than at its open end. The first piston arranged at the end of the piston rod is adapted to the housing in the section of the larger internal diameter, whereas the second piston is adapted to the section of the housing with a smaller internal diameter. In this way, the movement of the piston rod is determined by the second piston to the extent that it moves in the section of the housing with a smaller internal diameter.
The remaining stroke of the piston rod is influenced by the first piston. To influence the piston stroke, both pistons exhibit means for damping the movement. A piston ring capable of axial displacement is arranged on each piston between a supporting ring and a collar. The piston ring possesses a larger internal diameter than the area of the piston in which the piston ring is movably mounted. The annular gap formed in this way between the piston ring and the piston permits the fluid to flow between the pressure chambers. The annular gap is exposed or blocked by directionally-dependent contact of the piston ring against the supporting ring or against the collar. For the purpose of ensuring undamped movement of the piston rod, the first piston in addition exhibits an axially oriented bore, which is opened and closed by the axially moving piston ring depending on its position. The second piston possesses a locking device in addition to the axially moving piston ring. This consists of bores in the second piston, a valve plate for closing off the bores, a spring for pre-tensioning the valve plate, and a stop for the spring against the piston rod.
As the piston rod is extended, the piston ring of the first piston, because of its position, exposes an annular cross section between the first piston and the internal wall of the housing on the one hand and the bore on the other hand. The fluid is thus able to flow through the exposed bore and through the annular gap between the piston and the internal wall of the housing. Essentially no damping is perceivable in conjunction with this.
As soon as the second piston enters the section of the housing with a smaller internal diameter, the fluid flows through the internally situated annular gap in the piston ring and the supporting ring which retains the piston ring. The locking device is closed, so that no fluid is able to flow via the bores in the second piston. The extension movement is damped in this way.
As the piston rod is retracted, the fluid once again flows via the annular gap on the second piston. The locking device opens in addition, thereby enabling the undamped retraction of the piston rod in this section. As soon as the second piston emerges from the section of the housing with the smaller internal diameter, the further course of movement is determined by the first piston. As a consequence of the retraction movement, the piston ring closes the annular cross section between the first piston and the internal wall of the housing. The fluid is thus able to flow exclusively via the axially oriented bore in the first piston, as a consequence of which damping of the retraction movement of the piston rod is brought about.
A disadvantage associated with this displacement element is the intricate design of the piston. The second piston provided for the purpose of damping the extension movement with the locking device consisting of a plurality of components thus forms a complex unit, to be able to achieve the damping. End position damping, shortly before reaching the maximum extension position of the piston rod, is not possible in spite of the intricacy of the design. Damping of the retraction movement also requires an additional bore in the first piston.
Moreover, the displacement element is executed in such a way that the piston rod penetrates the housing on the side of the section with a smaller internal diameter. In order both to guide the piston rod and to seal the pressure chambers inside the housing from the atmosphere, this end must be closed with a sealing and guide package. For reasons associated with manufacture, the other end of the housing is also open. A consequence of this is that the second open end of the housing must be closed with an additional cylinder base in the end state. The cost of manufacturing the housing is thus also very high.